Star-branched polymers, also known as radial polymers, are characterized by having three or more polymeric arms emanating from a central core. These polymers can be prepared by various polymerization procedures such as anionic, cationic, and free radical mechanisms. The stars are usually formed by using either multifunctional initiators, multifunctional chain transfer agents, or multifunctional coupling agents. The star-branched polymers have unique properties including: narrow molecular weight distributions; low viscosities at low molecular weights or in solution due to their compact structures; high viscosities at high molecular weights due to extensive entanglements.
Heteroarm star polymers have been prepared using free radical methods involving partially capping a polythiol, typically by acetylation, grafting from the uncapped SH groups, removing the blocking group by hydrolysis, and then grafting the second monomer off of the liberated SH groups. U.S. Pat. Nos. 5,399,642 and 5,492,965.
U.S. Pat. No. 5,679,762 discloses the preparation of heteroarm star copolymers by free radical polymerization using sequential addition of monomers. This process relies solely upon the order of addition of monomers for its selectivity.
The present invention improves over the methods of the prior art by using chain transfer agents containing thiol groups with multiple reactivities. Thus, the selectivity is controlled by both the order of addition of the monomers and the composition of the polymercaptan core. Hence, greater control over the blockiness of the heteroarm star results.